The present invention relates to the control of laser beams; and more particularly, the invention relates to the detection of unwanted wave front distortions of light beams for the purposes of compensation.
Many applications of optical technology employ the principles of wave front analysis in order to acquire particular information. Wave front analysis can be applied to coherent as well as to incoherent light. Wave front analysis may be applied, for example, to where the radiation from a source is unresolved such as light emitted from a star or a star-like object. Wave front analysis can also be employed on laser beams.
Generally speaking, a laser beam as emitted has ideally equal phase in any plane extending transversely to the optical axis and direction of propagation of the beam. Such a beam may interact with an object, with another beam, or a combination thereof, and is returned, or otherwise intercepted, for the purpose of detecting any laser beam modulation, yielding information on the object with which the beam has interacted. Particularly, the interaction causes the wave front of the return beam to be modulated (in space), which modulation is also called optical path difference (or OPD, for short). If one considers any detection plane which extends transversely to the axis of the return beam, the several (complex) vectors describing the radiation in the different points in that plane oscillate no longer in phase. Detection of the phase difference yields relevant information on the object, etc., which has intercepted the beam earlier.
A problem arises if the laser beam undergoes deflections and other distortions along its transmission path, other than the desired modulation. In such cases, the wave fronts are no longer planar, even without any information modulation. Rather, distortions of the wave front are superimposed upon the desired modulation thereof, so that the detected modulation is rendered less reliable. Among such distortions, the following are, in order of importance, the most prominent ones.
There is first of all a deflection of the beam as a whole from the desired straight path. This deflection becomes noticeable as a tipping and tilting of any planar wave front relative to the optical axis in the receiving equipment because such axis assumes tilt- and tip-free wave fronts. It is customary to refer to such an inclination of the wave fronts in one direction as "tip" and an inclination in the orthogonal direction is called "tilt", whereby it is customary to assign the term "tip" to the most likely and/or most prominent deflection under the particular conditions of operation. However, these two terms could be used just to distinguish between inclinations on two orthogonal axes without preference or dominance of one over the other. Presently, it is more practical to use the terms "tip" and "tilt" without preference and to ascribe to each term a particular, fixed but arbitrarily chosen, tip or tilt axis of an x-y coordinate system.
Next in order of importance is a wave front distortion in which the center of the wave front of the beam runs ahead of or behind the more marginal portion of the front; that is to say, the wave fronts are spherically distorted, they "bulge". This type of distortion is called "focus".
A different type of distortion is called "astigmatism". The wave fronts bulge, but in different directions (convex vs. concave) in different planes. One can distinguish among the types of astigmatism, depending on the direction of those planes. There are other types of distortions; but they are clearly of a higher order and are, in effect, less noticeable in general when compared with desired wave front modulations by operation of the information as defined and to be represented by such modulation.
There is a need to detect such deflections and distortions in laser beams, particularly of the type defined above, and to eliminate them as much as possible, at least to the extent these interfering phenomena tend to falsify the desired information contained in wave front modulations. The task at hand is not so much the generation of theoretical conditions under which the distortions can be defined and eliminated. Rather, the problem is to find a practical solution which can be implemented without undue expenditures.